Add a dark mode to the website

[nt1m]

I like having all my websites dark as it reduces eye-strain :)

[jankeromnes]

Super cool idea! I already imagine going to settings to switch on the dark theme! 😄

Random thought about #223, maybe we could use the "black on white" icon for light theme, and the "white on black" icon for dark theme?

[ar5had]

@jankeromnes IMHO there is no big need to use 2 different favicons since the goal of dark mode is to reduce eye-strain.